1. Field of the Invention
The present invention relates to a futile single crystal with no grain boundaries (of large angles) and to a futile single crystal growth process by the edge-defined filmfed growth (EFG) method.
2. Description of the Prior Art
Rutile single crystals, known as polarizer material, are now produced as by the floating zone--FZ--method (cf. Japanese Patent Publication No. 61-101495) or Verneuil method. The crystals obtained by these methods are of about 10 to 25 mm in diameter, and have grown along their c-axis. In most cases, various forms of polarizers may then be made by cutting the single crystals--which have grown along their c-axis--at some angles with respect to the c-axis and further processing the plate crystals.
As well known in the art, on the other hand, the EFG method is a crystal growth method, by which some compounds inclusive of saphire and .beta.-alumina can be pulled up into single crystals conforming in shape to dies which may take on ribbon, round rod, cylinder or other desired forms. This principle is illustrated in FIG. 1. As illustrated, a melt 2 is filled in a crucible 1 incorporating a slit die 3. The melt 2 ascends through and along slits 4 (which may be poresin some cases) in the slit die 3 by capillary action, and reaches the upper face of the die 3, where it is seeded with a seed crystal 5. Then, while cooled, the melt 2 is pulled up at a constant rate to obtain a single crystal conforming in shape to the die. Note that reference numeral 6 represents a growing crystal.
As observed under a polarization microscope, crystals grown by the FZ or Verneuil method have so increased a temperature gradient on the crystal growth interface that they can easily include grain boundaries around or in them. The grain boundaries detectable by a polarization microscope are those of large angles ("Applied Physics", Vol. 46, No. 9, pp. 938-942). Portions of crystals containing such boundaries cannot be used as polarizer material. In order to process such conventional crystals into polarizers, etc., there is needed a step of removing grain boundaries of the large angles portions out of the crystals by cutting to obtain single crystals of good quality. The conventional crystals, as a whole, are not single crystals of good quality; that is, the yield of single crystals is too low to curtail material costs. These are major factors that make it unfeasible to reduce the cost of rutile polarizers and that render it difficult to cut down the cost of isolators as well, which are in great demand as optical communications are more prevailing than ever before.
To prepare futile polarizers with practically usable faces and morphologies, it is desired that crystals with desired morphologies be grown at a certain angle with respect to their c-axis, because this makes it achievable to simplify the later processings of the crystals with high efficiency and curtail the production cost of rutile polarizers. It is thus desired that rutile single crystals be grown at a certain angle with respect to their c-axis.
However, the thermal conductivity and anisotropy of rutile single crystals are higher in their c-axis direction than in other directions; that is, some difficulty is encountered in growing crystals in those directions by the FZ method or Verneuil method now usually available in the art. Although it is believed that EFG crystal growth is effective to this end, never until now is there any report that rutile single crystals have successfully be made by the EFG method. Essentially required for the EFG method is the material of a die through which a melt is delivered up by capillary action. In addition, that die material should not be subject to serious corrosion by the melt.
It is therefore one object of the invention to provide a rutile single crystal which, as a whole, can be used as polarizer material and processed into rutile polarizers more easily than ever before.
Another object of the invention is to provide a rutile single plate crystal growth process making use of the EFG method.